Refrigeration systems employing air at cryogenic temperatures for freezing food are commercially available, for example, see U.S. Pat. Nos. 3,733,848 and 3,868,827. In the latter patent, air is compressed in a first stage compressor, cooled in an intercooler, further compressed in a second stage compressor, cooled in another intercooler, further cooled by countercurrent exchange with cold air leaving the freezer and finally expanded in an expansion turbine mechanically coupled to the second stage compressor where the gas is reduced to about -180.degree. F. before being directed into the freezer. This prior art freezer has been used in combination with a vortex separator for removing particles of ice in excess of 5 microns in diameter from the air leaving the food freezer. However, despite this separation ice has been found to build up in the regenerative heat exchanger and to result in an intolerable pressure drop across the main heat exchanger which greatly reduces the efficiency of this system.